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The state's highest court ruled unanimously that a proposed amendment to the Oklahoma Constitution that would define a fertilized human egg as a person violates a 1992 U.S. Supreme Court decision involving a Pennsylvania case and "is clearly unconstitutional."

Supporters of the personhood amendment are trying to gather enough signatures to put it before Oklahoma voters on the November ballot.

Opponents contend the measure would ban abortions without exception and interfere with a woman's right to use certain forms of contraception and medical procedures, such as in vitro fertilization.

The American Civil Liberties Union and the New York-based Center for Reproductive Rights filed a protest with the state Supreme Court on behalf of several Oklahoma doctors and residents.

They asked the court to stop the group Personhood Oklahoma from gathering signatures.

The nine-member court determined the initiative petition "is void on its face" and struck it down.

"The only course available to this court is to follow what the United States Supreme Court, the final arbiter of the United States Constitution, has decreed," the court said.

The ruling is the latest setback for abortion opponents who have been pursuing personhood measures in several states.

In December, a judge in Nevada ruled that a personhood initiative petition was vague and could not be circulated for signatures to qualify for the 2012 ballot.

Similar personhood proposals were defeated last year in Mississippi and Colorado.

Dan Skerbitz of Personhood Oklahoma said the organization's attorneys will have to study the decision before deciding how to proceed.

He said that next year, supporters may ask the Legislature to place the issue on a statewide ballot.

The backers of the signature drive have said their goal is to set up a legal challenge to the landmark Roe v. Wade decision by the U.S. Supreme Court in 1973 that gave women a legal right to abortion.

"We're going to do everything we can to try to save innocent children," state Rep. Mike Reynolds, a Republican from Oklahoma City who is one of the petition organizers, said Monday.

Efforts to pass a personhood bill failed in the Legislature this year. A measure granting fertilized human eggs the rights and privileges of Oklahoma residents that had been approved by the Senate died in the House on Thursday.

House Speaker Kris Steele, a Republican from Shawnee, had said that a majority of the GOP caucus had privately voted against hearing the bill.

Opponents argued that the proposed amendment "would confer rights on a fertilized egg that trump the rights of each woman to determine whether and when to conceive and whether to carry a pregnancy to term."

Court documents filed by opponents noted the state Supreme Court rejected an attempt to use the initiative petition process to ban abortion in 1992, ruling that the process could not be used to curtail rights secured to women by the U.S. Constitution.

Nancy Northup, president and CEO of the Center for Reproductive Rights, said the Oklahoma Constitution specifically prohibits amendments that are "repugnant to the Constitution of the United States."

"This amendment would have run roughshod over the fundamental, constitutionally protected reproductive rights of all Oklahoma women," Northrup said.

Ryan Kiesel, executive director of the ACLU's Oklahoma chapter and a former Democratic lawmaker, said the court's decision Monday supports the notion that a woman and her family, "and not the government," should be making decisions on reproductive health care.